This invention relates to spark plugs for use in internal combustion engines, and is particularly concerned with the construction of the centre electrode of such spark plugs.
The electrodes of spark plugs operate under very unfavourable conditions and call for compromises as regards the various properties required in the materials used therefor. On the one hand good electrical and thermal conductivity is required, while on the other hand the material should have adequate corrosion resistance, particularly at high temperatures, to the reaction products resulting from combustion of the fuel, and a very slow burning rate is necessary in order to achieve a satisfactory service life. Furthermore the workability of the material must be such as to facilitate mass production, and a sufficiently high mechanical strength at elevated temperatures is required.
A consideration of these requirements will indicate, that pure metals or alloys are generally not capable of providing a favourable combination of the required properties. Centre electrodes made up of several different metallic materials have therefore been developed. However, the known arrangements suffer from various technical disadvantages and in many cases production costs are high. The present invention seeks to provide a centre electrode made up of several different metals which while providing the best combination of the desired electrode properties is inexpensive to produce.